1. Field of the Invention
This invention relates to an image forming apparatus and a method for calibrating density and color, and in particular, to an image forming apparatus such as a copy machine or printer and a method for calibrating density and color.
2. Description of the Related Art
Electrophotography is known as one of the image recording schemes used for an image forming apparatus such as a copy machine or printer. With electrophotography, latent images are first formed on a photosensitive drum using laser beam and then the image is developed with electrically-charged coloring material (hereinafter also referred to as “toner”). The image developed with the toner is then transferred and fixed onto a transfer paper and recorded as a final image. Recently, there are especially more tandem electrophotographic image forming apparatuses that have as many developing machines and photosensitive drums as toner colors and they sequentially transfer different color images into an image conveyor belt and a recording medium.
For the tandem image forming apparatuses, a known method prints specific patterns such as gradation patterns onto a recording medium such as paper after a warm-up process during startup in order to improve the stability of image quality. More specifically, the method reads printed gradation patterns with an image reading apparatus such as a scanner and then adjusts image forming conditions such as gamma calibration based on the patterns. However, one problem is the adhesion characteristic of the development toner against the potential of a photosensitive drum could change over time. As a result, the optimum image formation condition might not be maintained. To overcome the problem, traditionally, density calibration using the relationship between the potential data and density is performed to form monochromatically developed patches of C, M, Y, and K onto the non-image forming area of the photosensitive drum. The patches formed are read by a photo sensor. The output obtained by the photo sensor is converted with a predefined density conversion table, which value is then used for the density control of a LUT for one-dimensional gradation correction using gamma-calibration.
In another field of recent electrophotographic image forming apparatuses, especially in the field of quick printing industry, there is a need for much higher image quality. The color balance of a color image forming apparatus often changes according to process conditions such as laser exposure amount onto a photo conductor and development bias, and environment or chronological changes due to the heating of a fixing unit or the adjustment of pressurization temperature. For such changes, the calibration performed for the density control as described above would not be enough because even if the monochromatic gradation characteristics are adjusted with the above LUT for a one-dimensional gradation correction, a “mixed color” that requires a plurality of toners such as red, green, blue, and gray built from CMY produces non-linear differences depending on the type of the printer, causing difficulty in ensuring that correct colors will be produced. To overcome the problem, a technique is proposed that creates and outputs a chart with mixed colors in a range that a printer can reproduce, and measures colors with a scanner or color measuring unit, compares the measured color with a target value, and finally produces a corrected value (e.g., see, Japanese Patent Laid-Open No. 2006-165864).
In addition, another technique is proposed that focuses on a destination profile in an ICC profile and modifies the destination profile to correct color differences in mixed colors. Here, the ICC profile refers to data defined by ICC (International Color Consortium), which is used for converting colors. First, the technique outputs a chart of mixed colors with a printer, and then measures those colors with a scanner or a color measuring unit. The mixed colors can be corrected by calculating the difference between the measurement result and a target value and updating a three-dimension LUT (destination profile) that converts a device independent color space (L*a*b*) of the ICC profile into a device dependent color space (CMYK). The L*a*b* is one of the device independent color spaces, in which L* represents brightness and a*b* represents color phase and chroma, respectively. It has been examined that, utilizing this principle, mixed color patches on a recording medium are read by color sensors arranged on a paper conveyor path between a fixing area and an ejecting area to control density or color. Furthermore, a reference plate for calibrating the color sensors (hereinafter also referred to as “white plate”) may be arranged in front of each of the color sensors in order to calibrate the color sensors to improve the accuracy of their readings.
However, even with such a technique, when the color sensors or white plates are placed on the paper conveyor path between the fixing area and ejecting area within the image forming apparatus, the sensors and plates may be contaminated due to the exfoliation of tonners or dusts. In this case, a serviceman will be notified of the error and he must clean and replace both of the sensors and the plates. In handling such an error, if it is not clear which of the sensors or plates are contaminated, the serviceman will be required to replace and clean up both the sensors and plates, needing more time to perform replacement tasks and arrange parts necessary for the replacement resulting in frequent down times. A technique to overcome the problem is disclosed in Japanese Patent Laid-Open No. 2008-298854 in which an optical sensor reads optical density or the optical reflection characteristic of detection toner images formed on an intermediate transferring body. The technique of Japanese Patent Laid-Open No. 2008-298854 then forms reference image patterns of white coloring materials on the intermediate transferring body and corrects the optical sensor based on the reference image patterns. With this, it can be determined whether the color sensor or the intermediate transferring body is contaminated by reading the reference image patterns. In addition, any reference plates for calibration are not required. Furthermore, the color detection accuracy of the sensors can be improved for better image density control.
However, in Japanese Patent Laid-Open No. 2008-298854 above, a plurality of reference image patterns is formed using toners, the density of which is used to determine whether it is the color sensors or the intermediate transferring body that is contaminated. Therefore, considerable toners will be consumed accordingly. Furthermore, another problem arises in that reading errors of the color sensors will be caused leading to the degradation in the calibration accuracy of density and color because calibration using the reference plates is not performed at the initial stage.